Immobilization and leaching behavior of cobalt in alkali-activated fly ash/dehydrated cement paste fine blends

Abstract

The primary radioactive isotope in concrete waste from decommissioned nuclear power plants is 60Co, which must be securely immobilized and disposed of to protect human health and the environment. The leaching behavior of Co2+ in alkali-activated fly ash/dehydrated cement paste (DCP) blends was studied by focusing on the influence of different curing temperatures on phase evolution. When cured at 25 °C, alkali-activated fly ash/DCP blends predominantly formed the C-N-A-S-H gel phase, whereas at 60 °C, the C-A-S-H gel phase was more prevalent. By contrast, fly ash-based geopolymers without DCP predominantly form an N-A-S-H gel phase. The Co2+ ion leaching tests indicated that Co retention was most effective during phase evolution into the C-N-A-S-H gel. Conversely, fly ash-based geopolymers lacking DCP tend to form significant amounts of NaCl, which negatively impacts the Co retention. In these geopolymers, Co is mainly immobilized in the form of Co(OH)2.